A typical ceiling fan will include a downrod assembly suspended from the ceiling with a motor shaft connected to a lower portion of the downrod assembly. A motor body rotates about the motor shaft. A motor housing surrounding the motor may be secured to either the motor shaft or the downrod assembly, which is stationary. Blade mounting arms are connected to the motor body and extend out of an opening of the motor housing or below the motor housing. The fan blades are thus mounted as cantilevers extending from the hub. They have support bases typically no greater in area than the thickness of the blade times the width of the blade. Additional details concerning fans may be found in U.S. Pat. Nos. 6,250,885 and 6,354,801, issued to Gajewski and commonly assigned with this invention, which are incorporated herein by reference in their entirety.
In ceiling fan applications, the fan blades are relatively long to better circulate the air without creating concentrated drafts of air. The air resistance against the fan blade creates reactive moments at the proximal end of the fan blade, the end closest to the mounting rod. These reactive moments create stress in the fan blade that often leads to breakage. Additionally, ceiling fans are often run in both directions, e.g., both a winter mode for directing air upward and a summer mode for directing air downward. Running the fan in alternating directions can over time fatigue the fan blade. Such fatigue can also lead to breakage. To minimize effects of wind resistance, ceiling fan blades are typically set at relatively small angles-of-attack, relative to the direction of blade rotation.
Using small angles-of-attack, however, reduces the amount of air circulated when the blades are rotated. To compensate for reduced air circulation, the fans are often operated at higher speeds by an operator. Operating the fans consistently at high speeds can lead to a shorter motor life. High speed operation is also generally noisier, and hence more disturbing, than operating the fan at a relatively slower speed. Another problem often encountered with high speed operation is wobble and nutation of the fan hub due to, for example, an imbalanced rotor. Accordingly, it is desirable to generate an equal, or increased, amount of air circulation at reduced operating speeds. It is also desirable to reduce fan blade fatigue and breakage.